Interstellar Tours E-Book

Brian Clegg

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2023

Seitenzahl: 389

Beschreibung

'Strap in and enjoy the ride!' JOHN GRIBBIN 'A window seat on a flight to our galaxy's sites of outstanding beauty' MARCUS CHOWN, AUTHOR OF THE ONE THING YOU NEED TO KNOW 'A refreshing new look at our own corner of space' HENRY GEE, WINNER OF THE 2022 ROYAL SOCIETY TRIVEDI SCIENCE BOOK PRIZE 'Buckle up for the ride of a lifetime' PHILIP BALL, AUTHOR OF THE BOOK OF MINDS AND BEYOND WEIRD Take a voyage into space to explore the wonders of the galaxy and beyond. With award-winning science writer Brian Clegg as your deep space guide, step on board the starship Endurance and marvel at the fascinating sights of deepest, darkest space. Although our vessel is fictional, the phenomena you will visit, from the vast nebulae that are birthplaces of stars to stellar explosions in vast supernovas, creating the elements necessary for life - or from the planets of other solar systems to the unbelievably supermassive black hole at the heart of the Milky Way - all reflect the best picture current science has to offer. Accompanying Interstellar Tours is an online gallery with over fifty images and videos in full colour, each directly accessible from the page using QR codes. It may never be possible to undertake a voyage through the stars for real. But with Interstellar Tours, you can enjoy the ultimate cruise across the Milky Way.

Leseprobe

 

 

Published in the UK and USA in 2023 by Icon Books Ltd, Omnibus Business Centre, 39–41 North Road, London N7 9DP email: [email protected]

ISBN: 978-183773-075-9 ebook: 978-183773-077-3

Text copyright © 2023 Brian Clegg

The author has asserted his moral rights.

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher.

Typesetting by SJmagic DESIGN SERVICES, India

Printed and bound in the UK

For Gillian, Rebecca and Chelsea

CONTENTS

Acknowledgements

Pre-flight Checks

1Welcome Onboard

2Passing the Competition

3Stellar Nursery

4Other Worlds

5Supernova

6Pulsar

7Black Hole

8Collisions

9Milky Way

10The Big Picture

11Time, Life and Our Backyard

Appendix: Special Theory of Relativity for Beginners

ACKNOWLEDGEMENTS

Thanks, as always, to the excellent team at Icon, especially Duncan Heath.

I’d also like to remember those who got me interested in space and astronomy, many now sadly no longer with us. These include my father, Leonard Clegg, who drove with me halfway across the country to pick up the six-inch reflecting telescope that was my first experience of anything other than naked-eye astronomy. Then there was Patrick Moore for his eccentric but engaging commentaries on the long-running The Sky at Night TV show (and for his supportive answer to a letter of mine). Not to mention our only real space travellers to date, the astronauts who made it to the Moon, from Neil Armstrong to Eugene Cernan, who at the time of writing was the last person to have walked on the Moon – although hopefully this will not be the case for much longer.

It’s easy to think of space travel as science fiction – and it’s certainly the case that science fiction enables us to explore the universe (or, indeed, a whole range of universes) in our imagination. But I would like to hope that at some time in the future, even if not on as grand a scale as is possible for readers of this book, human space travel to the planets of our solar system and beyond will be more than just a dream.

PRE-FLIGHT CHECKS

A few years ago, I wrote a book called Inflight Science that described the science that we experience when on a plane flight, giving the reader a chance to explore the things we observe through the airplane window as well as the science and technology that makes the flight itself possible. Interstellar Tours is a sequel to Inflight Science that takes things to the next level. This is the science we might experience on an interstellar space liner of the 22nd century.

As much as possible, the facts are as solid as those featured in the pages of Inflight Science. However, cosmology is inevitably a very different science from the physics of our world. It features more speculation because we can’t do experiments or directly examine many of the phenomena that we are looking at from a vast distance. Where that’s the case, I will make this obvious in the text, but what we will experience on the flight is based on our current best understanding.

The only exception to this approach is in the first full chapter, Welcome Onboard. With our current technology, it is not possible to make a journey to distant stars. To be able to take our journey across the galaxy, we will discover the need to make use of technology that is pure science fiction after exploring and dismissing hypothetical technology that is based on current theory. Certainly, for the moment, there is no feasible way to make human interstellar flight practical.

Science fiction is sometimes treated as second-class literature – but I think it gives us a wonderful opportunity to ask, ‘What if?’ In recent years, the genre has been caricatured and denigrated, with Margaret Attwood even dismissing it as ‘talking squids in outer space’. But the real science fiction is about telling stories that explore the ways that humans might react to the impact of new science and technology on their lives and their environment. Although this book is set in outer space, there will be no talking squids – its role is to bring us closer to astronomical and cosmological features that would be on the tourist trail if only we could get out there – all based on the best current science.

Given that we won’t be meeting any talking squids, it’s worth reflecting why this is the case. Around 1950, the Italian physicist Enrico Fermi was taking part in a discussion around a canteen table at Los Alamos, the site of the US atomic bomb project during the Second World War. The subject of flying saucers (as Unidentified Flying Objects or UFOs were then known) came up. Fermi is reported to have said to his colleagues, ‘Where is everybody?’ It seemed strange that there was so much talk of aliens from space but that there was no scientific evidence for the existence of alien life.

For centuries there has been speculation about living beings (often all too predictably humanoid) existing on heavenly bodies. In the early days, this even included the Sun. And, since the 1940s, there was a steady stream of sightings of UFOs, or as we are now supposed to call them, unexplained aerial phenomena – UAPs. (They were renamed partly due to the US military’s love of novel abbreviations and, more reasonably, because quite a lot of these phenomena are not flying objects.) One of those present at the Los Alamos discussion remembers seeing a cartoon at the time explaining that the disappearance of trash cans from New York City was the work of little green men who had been taking them away. Aliens were very much part of the zeitgeist. But they were hardly knocking at the door of the White House.

Although we can’t be certain, there is no credible evidence that UAPs have any connection with visitors from other planets. As has been widely pointed out, given that so many individuals now carry high-quality cameras with them at all times, built into their phones, it is strange that we still only get vague fuzzy photographs to support claims of UAP sightings. More so than ever, we can now ask, ‘Where is everybody?’

It seems unlikely, then, that we have been or will be visited by aliens. There certainly may be alien life out in the galaxy (though if there is, the chances are that the majority of it is more similar to bacteria than humans), but we don’t have any good scientific evidence to back up its existence. The old saying ‘the absence of evidence is not evidence of absence’ is technically true – but it is simply telling us that, for now, science, which needs to be evidence-based, has nothing useful whatsoever to say about alien life. All we have is pure speculation, unsupported by evidence, or stories.

I’m not knocking aliens in science fiction – there are some great characters, though the ones we develop affection for, from Mr Spock to Yoda, tend to be far more anthropomorphic than anything we might expect real aliens to be like. The reality is that fictional aliens are often playing a role that helps us explore what it is (and what it isn’t) to be human. But until we have any scientific evidence, fiction is where they belong. That being the case, aliens will not be making an appearance on our interstellar tour.

There may be no aliens on our journey into outer space, but what you will be able to see is an impressive collection of images, provided on the Interstellar Tours website: www.interstellartours.co.uk. Where possible, these are photographic, but any featuring fine detail of distant systems, planets and solar system will be artists’ impressions. The aim is for them to give you a sense of what it is like to experience a visit to these locations, but it is important to bear in mind that the exact detail shown may not be correct.

For the moment, though, it’s time to take our place on board the starship Endurance* and to get a quick introduction to the technology that is going to be necessary to make our voyage possible, and survivable.

* Our starship is named after Ernest Shackleton’s wooden ship, lost in 1915 when it became trapped in pack ice when exploring Antarctica. We can never overemphasise the influence of the 20th-century TV show Star Trek on the interstellar touring business. Our entire starship fleet has three syllable names beginning with E.

WELCOME ONBOARD 1

One thing that the classic TV show Star Trek (mostly) got right is that starships don’t land on planets. It’s easy to underestimate just how difficult it is to get a massive object off the surface of a planet and into outer space. The problem lies in escaping the gravity well. An object as big and heavy as the Earth – which has a mass of around 6x1024 kilograms (13x1024 pounds) – holds on to objects on its surface with an iron grip. Even when birds or planes do make it into the sky, they soon have to return to the surface. What goes up really does usually come down.

Units and stuff

Occasionally we will be using scientific notation like the 6x1024 above. This is just a convenient way of representing large numbers. Here, 6x1024 is shorthand for ‘6 multiplied by 10, 24 times over’ – or to put it another way, 6 followed by 24 zeroes. You could also say that it’s 6 trillion trillion.

Science makes use of the metric system for all measurements, and, by the time the Endurance was commissioned, no one on Earth was still using the traditional units such as feet or pounds. They had gone the same way as rods, poles, perches, bushels and chains as units of measurement. However, for old times’ sake, we will show both metric and traditional ‘Imperial’ units, except for restricting weights to metric tonnes, as these are close enough to traditional tons to make the distinction unnecessary.

We are used to measuring long distances in kilometres or miles, but in space, a kilometre is a pathetically small unit. The most useful measure for us will be the light year – the distance that light travels through space in a year. A light year is 9.46 x 1012 kilometres or 5.88 x 1012 miles. To put that in context, the distance from the Earth to the Sun is about 8.3 light minutes or 0.000016 light years. Astronomers often prefer to use distance units called parsecs (which are around 3.26 light years). These work particularly well with the mechanics of telescopic observations, but we will stick to light years as they are easier to envisage.

Are you massive?

Because we are going to spend our time during the journey out in space, it’s worth quickly clearing up the distinction between mass and weight, because the difference matters very much when you are away from the surface of the Earth. These terms tend to be used interchangeably back home, but they are very different things, and in space this will become obvious.

Mass is an intrinsic property of an object, which is measured in kilograms (officially, the traditional mass unit is called a slug (14.59 kilograms), although the pound tends to be used more often). It doesn’t matter where an object is, it will always have the same mass, unless bits are removed from it or added to it. You could see mass (a concept introduced to the world by Isaac Newton back in 1687) as a measure of the amount of stuff in an object – whether that object is you, a starship or something as large as the Earth.

Weight, by contrast, is the force that is felt by an object under the gravitational pull of a body such as a planet. When we talk about the weight of something, we really mean ‘its weight when it is on the surface of the Earth’, though we tend to omit the last bit. Your weight would be totally different if you were on the surface of the Moon, for example – about a sixth of what it is on Earth. In space, your weight could be zero, though, as we will discover, it certainly doesn’t have to be, and it will only be zero on the Endurance when in a special, low-gravity entertainment area. Having weight makes doing many things much easier – from eating to visiting the toilet.

Although your bathroom scales will give you your weight in kilograms or pounds or stones, this is a cheat. Strictly speaking, weight is the force due to gravity acting on the mass of an object. Scientifically, this should be measured in units called newtons (foot-poundals for traditional unit fans), but in practice we tend to fudge it and still use the mass based on what’s measured on the Earth’s surface. So, when we say that that on the Moon you will weigh one-sixth of what you do on Earth, what this means is that you will feel the force pulling you down that you would experience if you had one-sixth of your mass on the Earth’s surface.

Whether we talk of mass or weight, the first stage of taking our interstellar tour is getting off the Earth. And here things have not moved on as much since the earliest days of space flight as 21st-century people might have expected. The old rockets were extremely unsafe and scary. Nonetheless, we are still using the equivalent of rockets, although with far less risky sources of propulsion, and the ability to make the journey into orbit under a level of acceleration that won’t put the stresses on the body experienced by early astronauts. For us, it’s no different from taking a plane trip. We have moved on from the first journeys into space, but not as much as science fiction writers might have hoped for.

As for the Endurance itself, nothing as massive as a starship could survive landing or take-off from a planet. The ship was assembled in space with materials originating from Earth, the Moon and mined asteroids. The Endurance is a native of space itself.

No easy getaway

Using a traditionally fuelled rocket to get off the Earth was both expensive and risky. There are two ways to get an object into space. You can throw it, or you can push it. In practice, we usually go for the latter, but first it’s worth taking a look at the former.

If you can throw something faster than ‘escape velocity’, it will get away from the Earth’s gravitational pull and not return. If you had a suitable superhero to help you out, they would have to throw a ball straight upwards at 11.2 kilometres (seven miles) per second for it to reach this speed. That’s extremely nippy. The fastest fighter jets of the early 21st-century flew at around three times the speed of sound, but the ball would need to travel eleven times faster than this.

There is a way to cheat a little, because helpfully the Earth is rotating and we can make use of that. Something that is shot off the Earth in the right direction does not have a standing start, because it is already travelling at the speed of rotation of the Earth’s surface. By piggybacking on the Earth’s movement, we can get escape velocity down to around 10.8 kilometres (6.7 miles) per second – but that is still ridiculously fast. This, incidentally, was the approach taken by one of the first science fiction space flight stories, Jules Verne’s From the Earth to the Moon (De la Terre à la Lune)*.

In his novel, Verne’s adventurers were shot from the Earth using a 274-metre (900-foot) long cannon called Columbiad. The distinction between a cannon and a rocket is that the projectile in a cannon is only being accelerated while it is in the barrel. As soon as it leaves, it can only get slower. Unfortunately, to get a capsule up to escape velocity by the time it had traversed the Columbiad barrel would have required so much acceleration that the astronauts would have been mashed into jelly. Even if Verne had stretched Columbiad to ten kilometres (6.2 miles) in length, those on board would have suffered 600 times the force of the Earth’s gravity. The acceleration they endured would be vastly more than the around 9g* that is about the most a human can survive.

Yet we’ve all seen video of rockets taking off and getting away from the Earth. They seem to climb very ponderously into space. Although some of the slowness is illusory, they don’t fly at anywhere near escape velocity. The reason they can travel relatively slowly and still get up into space is that they aren’t thrown like a projectile from a cannon. All the time that the rocket motor is active, the capsule is being pushed. And as long as the force of that push is bigger than the force of gravity pulling the spaceship down, you can travel as slowly as you like away from the Earth. It’s the same as riding up in a lift. We don’t need to go quickly; we just need to have enough upward force to overcome the downward pull of the Earth.

The catch, though, with rockets is that the more mass the object has, the more fuel it takes to keep it moving long enough to escape the Earth’s gravity well. And every drop of fuel you have onboard adds to the mass. So that takes even more fuel. This is why rockets that carried any sizeable payload used to have multiple stages. (This was before the use of modern nuclear or antimatter-powered orbital shuttles.) That way, once the fuel in one section is mostly used up, a large chunk of the mass could be dropped off in the form of a stage, leaving far less mass for the remaining fuel to propel. Add in the fact that traditional rocket fuel is highly inflammable and potentially explosive and it can be seen that using a straightforward rocket to get into space was always a last resort.

That stages would be necessary for manned flight was publicised by Russian rocket theorist Konstantin Tsiolkovsky as early as 1903, the year that the Wright Brothers first flew an aircraft. While we’re dealing with rocketry, it’s also worth mentioning that to begin with, a number of respectable (if scientifically ignorant) figures doubted that rockets could work at all in space. Back in 1920, US rocket pioneer Robert Goddard published a paper entitled ‘A Method of Reaching Extreme Altitudes’ that suggested a rocket might be used to get to the Moon. The New York Times could not resist teasing Goddard in an editorial published on 13 January 1920 for what the article’s writer believed was a silly error:

That Professor Goddard, with his ‘chair’ in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react – to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.

What The New York Times misunderstood when it shot itself in the foot with this article was the meaning of the ‘equal and opposite reaction’ bit in Newton’s third law of motion (him again). The rocket does not somehow press against the atmosphere and get pushed forward in the resultant reaction. Instead, it pushes out its exhaust and the equal and opposite reaction is that the rocket is propelled forward. This can happen just as well in a vacuum as in an atmosphere. Better, in fact, as there is no air resistance to hold the rocket back.

Rockets definitely do work in space. The New York Times has been proved wrong many times since the 1920s (and the newspaper did issue a belated ‘correction’ in 1969 when Apollo 11 was on its way to the Moon). But 20th-century rockets were still clumsy, costly and dangerous. Back then, there was a lot of excitement in theoretical space travel circles about space elevators, particularly after they were used as a central feature of Arthur C. Clarke’s 1979 novel The Fountains of Paradise. Before we get to elevators, though, we need to understand what an orbit is.

Orbital velocity

Orbits will feature regularly on our exploration of space. Understanding orbits is not particularly difficult, but they are counter-intuitive. When a spaceship, say, is in orbit, it is in freefall towards the planet or other body it is orbiting. It would be dropping straight downwards to crash on the surface were it not also moving sideways at just the right speed so that it keeps missing the planet. The outcome is that it travels around the planet at a constant height.

For any altitude above the planet’s surface, there is only one speed at which the orbiting ship can travel. If it went any faster, it would fly off into space – and if it went slower, it would spiral its way down and collide with the ground.

One particularly useful orbital speed is to travel at the same rate that the planet rotates. This keeps the spaceship (or satellite, for example) at the same position over the Earth. When satellites are in this orbit around the Earth they are called geosynchronous*. Rather than moving through the sky as seen from the planet’s surface, the satellites stay over the same point at all times. This can be useful, for instance, for communications purposes. To stay in place, the satellite needs to be positioned 35,786 kilometres (22,236 miles) above the Earth’s surface. To put that distance into context, the circumference of the Earth is around 40,000 kilometres (24,900 miles). In fact, the kilometre was originally defined as 1/10,000th of the distance from the North Pole to the equator through Paris. So, a geosynchronous satellite has to travel upwards by nearly as far as a journey around the Earth.

Once we can imagine a satellite in a geosynchronous orbit, we have the starting point to imagine building a space elevator.

Climbing an elevator to heaven

The name sounds so simple – a space elevator is a lift that we can ride up into the sky to reach orbit. If we could set this up, there would be no need for our ascent vehicle to be laden with fuel. Let’s imagine we had a nice, chunky geostationary satellite and dropped a cable from that down to the surface of the Earth. Then we simply create a vehicle to begin climbing up the cable, powered by electricity provided from that cable, so the elevator has no need to carry fuel onboard. It’s a neat solution, but there are one or two problems.

Before we get into the detail, it’s worth saying that the space elevator wouldn’t get us entirely away from Earth. But at 35,786 km (22,236 miles) up, the pull of the Earth’s gravity would be reduced to about 1/44th of the level on the surface – this means it would take very little effort to escape its pull. (Our starship also has to leave the Sun’s gravity as well, requiring some more energy to be expended, but an even smaller amount.)

A first problem with the technology is that a space elevator would be an extraordinarily slow way to start a journey across the galaxy. Remember, heading up the elevator would be the equivalent of taking a journey almost around the Earth’s circumference. It would feel decidedly slow, as we tend to underestimate the distance involved. If the elevator travelled at a reasonable 200 kph (124 mph), it would take about 7.5 days to make the climb.

However, the bigger issue the designer of a space elevator faces is the strength and mass of the cable. One immediate problem is that as soon as we suspend a cable from the satellite, the combined body is no longer orbiting at the right height. As Newton realised, something orbiting acts as if all its mass is in a position where there is the same amount of mass in all directions. We would need to find this centre of mass for the satellite plus cable combined – which would be below the orbital height of the original satellite. In practice, the satellite would need a big counterweight above it to counter the mass of the cable below and keep its centre of mass 35,786 kilometres (22,236 miles) above the surface.

As for the cable, its mass would be considerable. Let’s assume that it is about 28 millimetres (just over an inch) across. This would enable it to carry around 50 tonnes of load, which would probably be enough for the size of elevator that we need to haul people and freight up to our starship. Unfortunately, though, 35,786 kilometres of this cable would have a mass of around 115,000 tonnes, which would mean that it would be incapable of supporting its own weight.

Even with the strongest, lightest material currently available – the atom-thick carbon-film layers called graphene – it would not be practical to make an Earth-based space elevator. And although material science has moved on since the 21st century, we still have nothing strong enough.

However, given the Moon’s much lower gravity it might seem that it would be useful to build a space elevator there, enabling sections of our starship to be constructed under the lower gravity on the surface of the Moon and hauled up into space. Unfortunately, though, the chances are that such an elevator would not work on the Moon either. Although there is less gravity to give the cable weight, it would have to be longer at around 88,000 km (54,700 miles) to stay in position. To make matters worse, it could only be located on the far side of the Moon: otherwise, the relative closeness of the top of the elevator to the massive Earth would make the whole structure gravitationally unstable.

There are alternatives to rockets to get cargo off the Moon, though. The starship Endurance was in part assembled from sections flung up into space from the lunar surface using a mass driver. This is a device that uses electrical energy to accelerate the cargo down a long track, building up enough speed to achieve the Moon’s relatively low escape velocity of 2.38 kilometres (1.48 miles) per second.

Beam me up

As we’re getting our heads around the science of starships, it would be remiss not to consider the possibility of using something equivalent to a Star Trek transporter to get off a planet. This idea was not originally based on science, but rather on the TV show’s budget. The producers couldn’t afford the time and effort to produce the special effects required to show a shuttle landing and taking off every time the crew visited a planet, so opted for a magical instant fix in which people were ‘beamed up’. But is this kind of transporter scientifically possible?

The only scientific avenue that might be able to help is what’s known as quantum teleportation, which effectively means making an identical copy to a quantum particle in a remote location. But the reality of using this approach falls down due to the sheer scale of everyday objects when viewed at the atomic level.

Think, for example, of what would be needed to transport a human being. If you are an average size, you will have around 7x1027 atoms in your body. To use some form of teleportation, you would have to somehow scan every atom in your body (including its structural links) and reproduce them all at the destination. There is no clear way to do this. But even if there were, it’s problematic. Let’s imagine you could do this at, say, a trillion atoms a second. This sounds impressive, but it would take around 200 million years before your journey would be completed.

There is also the minor problem that transporting atom by atom doesn’t mean that the finished item is then magically reassembled, whether it’s you or part of a starship. And if it were you, even if the process could be made to work, and such a teleportation device could make a perfect copy at the remote location, the original person would be destroyed in the process. After passing through the transporter, you might get something indistinguishable from the original Captain Kirk, say, but from his viewpoint, each time he beams down or up, he dies. It does not sound an appealing prospect.

The conclusion is that the materials to build our starship need to be shipped up into orbit using conventional propulsion, as would any supplies and passengers, though as already mentioned, this is now more routine – far safer and less dramatic than rocket launches in the early days. All we have to do once we are pretty much out of the Earth’s gravity well is to keep our passengers comfortable and to deal with the entirely non-trivial issue of travelling faster than the speed of light. But first, let’s take a look at staying comfortable.

Defying gravity

We are so used to living in the vicinity of a massive object – the Earth – that it’s easy to lose sight of how important gravity is to us from the point of view of both health and comfort. Before interstellar travel became possible, the most familiar trips were to the Earth’s orbit, the Moon and Mars. Earth orbit might have seemed impressive when it was first achieved, but it is not a true space journey. If we look, for instance, at a famous orbiting destination of the distant past, the International Space Station (ISS), visitors onboard that makeshift craft felt that they were experiencing zero gravity – but in reality, they were in free fall.

As we’ve already seen, an orbit is a balancing act between falling and travelling sideways in order to keep missing. Anyone in free fall under a gravitational field does not gain any weight from that field – they feel that they are floating. But that doesn’t mean that they are not experiencing gravitational attraction. The old ISS orbited at a mere 350 kilometres (218 miles), give or take, above the Earth, which means that it experienced around 90 per cent of the gravity at the planet’s surface. It was only because astronauts were constantly falling that they felt weightless.

On the Moon or on Mars, by contrast, the situation is much the same as on Earth – standing on the surface, travellers feel a constant gravitational pull from the nearby massive body. As we’ve already seen, that is around one-sixth of the Earth level on the Moon and it’s about two-fifths on Mars. Once you are on the starship, though, and well away from a planet or star, there will be no natural gravitational pull.

For a short time, this can be enjoyable. Sections of the Endurance are left without gravity so that passengers can experience floating around and can play three-dimensional sports. However, there are good reasons to avoid staying without gravity too long. Some find the experience induces nausea, and no one enjoys the requirement to use a bathroom with no gravity to help things progress naturally. More importantly still, the human body evolved to exist in a gravitational field of around 1g. It struggles in zero g. Muscles start to waste away, while bones lose density, making them more fragile. Long exposure to low gravity would mean that our lungs would become less effective because the diaphragm in the chest shifts up and the liver floats upwards, leaving less space for breathing.

It’s not just humans (and other animals) that deteriorate under low gravity. Plants get confused and don’t grow as well as usual because they use gravity to point their roots in the right direction. Way back, on one of the space shuttles originally used to access the ISS, it was discovered that a batch of fertile quails’ eggs failed to hatch without gravity, which it seemed was necessary to pull the egg yolks to their proper position near the bottom of the shell to spur on the development of the chick. Amusingly, this experiment was sponsored by the then popular fried chicken fast-food company KFC.

The importance of gravity for the comfort and safety of passengers means that a ship like the Endurance needs to provide some form of artificial gravity. Science fiction usually provides this as a convenience at the flick of a switch with a hand-waving description of an ‘artificial gravity field’.* In reality, there is only one effective approach to producing artificial gravity, which is based on Albert Einstein’s equivalence principle.

This concept was the inspiration behind Einstein’s general theory of relativity, which describes how massive bodies produce the effects of gravity by warping space and time. Thankfully, we don’t need to get into too much depth on this theory (yet – we’ll need a bit more when we encounter some of the more dramatic natural space phenomena). But the important realisation that Einstein had – something that he described as his ‘happiest thought’ – was that someone in free fall, for instance in a falling lift, would not feel their own weight. The acceleration they experience is equivalent to a gravitational pull and the two cancel each other out. This, as we have seen, is why astronauts are weightless in orbit.

Turning the effect on its head, if you accelerate someone, they will feel the effect of a gravitational pull. When, for example, a car or plane accelerates off at a high rate, you are pushed back into your seat. The sensation is just the same as being pulled towards the Earth by gravity – the two are indistinguishable. If you accelerate someone in space with a force of 1g, producing the acceleration they would experience due to gravity at the Earth’s surface, they will feel that they have their normal weight.

In principle, then, it’s easy enough to generate gravity – just keep the ship under constant acceleration of 1g. But as we will discover, conventional acceleration isn’t involved in the way that a starship’s drive works. And, even if the two types of motion were compatible, it would take a vast amount of fuel to keep accelerating at 1g for weeks or months at a time.

Luckily, there is a particular kind of acceleration that does not require energy input once it has been initiated. This is the acceleration that is involved in spinning around.

You spin me round (like a record)

When we think of acceleration, it’s usually a matter of getting faster (technically, it can also be getting slower) – but all acceleration means is experiencing a change in velocity. The ‘v’ word is not just an alternative term for speed. Speed tells you how fast something is moving, but velocity incorporates both speed and the direction of motion. Physicists call a quantity that has both size and direction a vector (as opposed to something like speed that only has size, which is called a scalar).

Because velocity is a vector, its value changes if we change direction, even if we carry on moving at the same speed. It makes sense if you think that acceleration is caused by giving something a push (applying a force in physicists’ terms). That can just as much be done to change direction as it can to speed something up or slow it down.

As a result, we can generate artificial gravity by spinning something around. You may have experienced this on a fairground ride. There’s an attraction that was popular in the old-style travelling fairs that is made up of an enormous vertical drum that you stand inside. The drum spins around, then the floor drops away – but when it does so, you remain pinned to the wall rather than falling. Acceleration’s equivalence to gravity is generating an artificial gravitational pull towards the outside of the drum.

Something else you might have experienced while on a ride like this is a feeling of nausea. Your sense of balance is controlled by a system that involves fluid in the inner ear. This system is often confused by motion, particularly acceleration – producing the symptoms of travel sickness. Being spun around is a great way to produce nausea – something you wouldn’t want to experience on our expensive interstellar tour.

Luckily, there is a way to spin without feeling bad. This is just as well – after all, the surface of the Earth at the equator moves at about 460 metres (1,500 feet) per second – over 1,600 kilometres (994 miles) per hour – yet we don’t notice it. Make the rotating environment big enough and it doesn’t generate that sick feeling. Obviously, the starship can’t have the same diameter as the Earth – but that isn’t necessary. And for that matter, we don’t need the whole ship to rotate either.

The Endurance looks nothing like most of the spaceships of sci-fi movies. They tend to be slick and streamlined. But streamlining is only relevant to the design of a ship that is required to pass through an atmosphere. In space, there is no air resistance. The clumsy-looking Discovery One in the remarkably forward-looking 1960s film 2001: A Space Odyssey was probably closer to the real thing than anything else that has been seen in the movies. That ship even had a rotating section to produce artificial gravity for exercise*, though the diameter of it was too small to have worked in reality.

To get a full 1g of artificial gravity without the passengers experiencing nausea, the wheel would have to be about 500 metres in diameter. On the first interstellar liners, which used this technology, a compromise was made at 0.5g, giving sufficient artificial gravity to avoid significant muscle wastage, but making the wheel section more manageable in size at a diameter of 100 metres (330 feet). Even so, the design did give such ships a significant bulge in the mid-section. The Endurance, being the most modern in the fleet, has a smaller mid-section bulge, but the gravity provided there is not artificial: it is produced by neutron star technology, which makes it possible to avoid rotation altogether.

Speculation alert

As we take a trip on the Endurance, you will encounter occasional ‘Speculation alerts’ like this one. With the exception of the drive system, which is purely fictional, these highlight when you see or experience something that is possible based on a 21st-century understanding of science, but that may not haveproved actually true or feasible by the time the Endurance was launched.

Unlike on earlier ships, the Endurance’s gravity section is not rotated. Instead, it is a large cylinder with a long, extremely thin core of neutron star material threaded through its centre. (We will be visiting neutron stars later in the voyage.)

This gives the different cylindrical floors positioned around that core a gravitational pull equivalent to between 1 and 0.2g, depending on the level’s distance from the centre. Extracting neutron star material was an interesting challenge to say the least – but given interstellar flight technology, it is not scientifically impossible.

Tea, Earl Grey, hot

Few can think about starship design without reference to that 20th-century exemplar, Star Trek’s USS Enterprise. In the ‘Next Generation’ incarnation of the show, much was made of the replicator, which was used to produce food and drink, including the preference of Patrick Stewart’s Captain Jean-Luc Picard for ‘tea, Earl Grey, hot’.

The ‘hot’ part of this command caused some controversy, as no one would sensibly drink Earl Grey tea cold. It seems hard to believe that a ship featuring technology that could produce food and drink apparently out of thin air would not have a sufficiently clever AI to realise that when someone asked for Earl Grey tea, they wanted it hot. But what is often missed in criticising the vison of the Star Trek creators is how much the replicators were under-utilised. Why could they make food and drink but not, say, uniforms or weapons or parts for engineering?

There certainly is an advantage in a starship being able to carry raw materials and construct most consumables on board, rather than having to carry a huge range of stores. Unlike an ocean liner, a starship can’t rely on supplies being available at every port. In reality, there are very few locations taken in by a trip like that undertaken by the Endurance where there will be any outposts of humanity from which to pick up essentials.

The practical precursor to the replicator, first introduced in the 20th century, was the 3D printer. Initially limited to plastics, during the 21st century, their capabilities were expanded to include everything from food to metals, in applications that ranged from tiny mechanical parts to full-size buildings. The 3D printer is still a staple for producing objects on the Endurance, but to get to a Star Trek-style replicator required inspiration from a concept put forward by the American engineer K. Eric Drexler.

It seems that Drexler was himself inspired by the great American physicist Richard Feynman. Although Feynman was best known for his work on quantum physics and on the commission that investigated the disaster that occurred in the flight of the Space Shuttle Challenger, he also explored the science of manipulation of the very small in a 1959 lecture ‘There’s Plenty of Room at the Bottom’.

Feynman envisaged using extremely small manipulators to construct even smaller manipulators, which then worked on the next level and so on, until devices existed that could interact with individual atoms to construct anything from the appropriate chemical elements. This wasn’t intended as a practical piece of technology – Feynman was aware of the physical challenges involved – but he put it forward as a provocation to think differently. This was evidenced in a couple of challenges that Feynman threw out to the audience: to build a motor smaller than 400 micrometres (0.016 inches) on each side and to produce text small enough to fit the entire Encyclopaedia Britannica on a pinhead.

Remarkably, the first of these challenges was achieved the next year (although using conventional engineering methods), while the second was cracked in 1985. But neither went to the extreme that Feynman envisaged. In his 1986 book Engines of Creation, Drexler came up with a more detailed description of how nanoscale manipulation of materials could be undertaken with a collection of tiny robots called assemblers that could piece together materials atom by atom.

Just as the Star Trek transporter has a scale problem, so did Drexler’s vision. There would have to be vast numbers of assemblers to do anything realistic. Imagine you had sufficient assemblers to put together, say, 7,000 trillion atoms per second – it would still take 200 years to construct Picard’s cup of tea.

Speculation alert

It is entirely possible that the kind of assemblers envisaged by Drexler, using an engineering approach, will never be physically possible to construct. The forces and interactions between objects on the scale of atoms are totally different to those we experience in the everyday world. However, nature makes use of ‘molecular machinery’, for example, in manipulating molecules in the production of proteins using the templates provided by genes. Assemblers are likely to be possible at some point to create pseudo-organic materials, but may not be able to produce, say, a piece of electronics. Biological processes can also take a considerable amount of time to complete an assembly (think how long it takes to produce a fully grown human), so such biological assemblers are likely to be limited to small products when working on a short timescale.

On the Endurance, a mix of onboard stock and 3D printing is complemented by the use of biological assemblers for small quantities of essential substances, such as medication and the flavouring used to give a relatively convincing facsimile of familiar foodstuffs. As yet, no one has ever requested Earl Grey tea, hot or otherwise.

The light speed barrier

It’s not enough to survive and thrive in space – a starship, by definition, must be able to reach the stars. And it is in the physics of high-speed motion that we hit the biggest physics problem for interstellar travel: relativity. Einstein’s special theory of relativity makes it clear that there are some limitations of movement through time and space, all tied into the limiting speed of light in a vacuum.

At the start of the 20th century, it had already been established that in order for light to exist, it has to travel at a fixed speed in any particular medium*. This makes it different from anything else. If two cars travel towards each other, both moving at 50 kph (31 mph) we add their speeds together and say their relative speed is 100 kph (62 mph). But however fast you move towards or away from a beam of light, it will always travel at the same velocity. In his special theory of relativity, Einstein established an implication of this fact when combined with basic physics: if something moves with respect to an outside observer, that observer will see the object’s time slow down, its length shrink and its mass increase.

It may seem extremely unlikely to be able to get to this finding from the simple fact that light always travels at the same speed, but the maths involved requires little more than Pythagoras’ theorem and can be followed easily by a high-school maths student. We won’t go through the details right now, but they are included in an appendix at the end of the book for those who are interested. We will also come back to relativity and its implications for time travel towards the end of our trip.

The limitations of relativity mean that in practice it is very difficult to get a spaceship up to anything near the speed of light. And it is physically impossible to accelerate past the speed of light, which presents starship designers with a problem. Let’s say we manage to get a ship up to a respectable quarter of the speed of light. From the viewpoint of those on Earth, it would still take around sixteen years to reach the nearest star to Earth other than Sun (Proxima Centauri) and another sixteen years to return.

There would be some slowing of time due to special relativity – but from the viewpoint of the travellers, at this speed, the round-trip journey would involve spending over twenty years onboard the ship. Even if the starship could travel so close to light speed that time hardly passed at all while travelling, a full eight plus years would have elapsed on Earth when they returned, meaning all their friends and relations would have aged in comparison to the travellers.

As it happens, Proxima Centauri will be on our itinerary a little later, but to get the true Interstellar Tours experience we need to travel much greater distances, which means being able to fly much faster. Sub-light speeds would not hack it. But there is a potential solution – because, contradictory though it sounds, although it is not possible to pass through space at greater than the speed of light, it is entirely possible (in theory) to get from A to B at much greater than light speed.

Let’s do the warp drive again

Even today, a whole lot of scientists are Star Trek